A suspension bushing of vertical type that establishes a connection between a body of a motor vehicle and a suspension arm in a vibration isolation manner is publicly known. It is comprised of an inner cylinder and an outer cylinder provided coaxially and spaced apart from each other in an inner and outer direction, and an elastic member for elastically coupling these inner and outer cylinders, and an axis of the suspension bushing extends in an upward and downward direction of the vehicle. This type of the suspension bushing is disclosed in U.S. Pat. No. 5,938,220, for example.
Such suspension bushing of vertical type has a tendency to increase a twist that the inner cylinder and the outer cylinder are inclined relative to each other. When the twist spring that is the spring in a twisting direction is increased in spring, it is easy to feel a stiff feeling thereby to easily decrease a riding comfort. Therefore, to cope with this twist, there are some cases where the spring in a direction orthogonal to the axis is strengthened by expanding an intermediate portion in the axial direction of the inner cylinder into a spherical shape and the twist spring is decreased by forming annular recesses of a hollow concave shape on axial end portions of the elastic member, thereby to improve the riding comfort.
Such suspension bushing is disclosed in U.S. Pat. No. 5,887,859 though there is no description about the vertical type, for example. FIG. 7 shows the same structure as a suspension bushing described in this patent document. This suspension bushing 120 is comprised of an inner cylinder 122, an outer cylinder 124, an intermediate plate 126, and an elastic member 150, 152 for functioning as a vibration isolating core element by elastically connecting these component elements.
A reference character 115 denotes a mounting portion provided on a suspension arm. The suspension bushing is mounted by press-fitting the outer cylinder 124 into a mounting hole 116 which passes through the mounting portion 115. A bolt (not shown in the drawing) is fitted into an axially extending through hole of the inner cylinder 122 thereby to be mounted on the vehicle body (not shown).
In a central region in an axial direction of the suspension bushing 120, there is provided an expanding portion 130 which projects outward in a radial direction. On the intermediate plate 126 corresponding to the expanding portion 130, there is provided a curved surface portion 140 which faces the expanding portion 130 and has a concave curved surface for accommodating the same. On both axial end portions between the inner cylinder 122 and the intermediate plate 126, annular recesses 154 of a concave shape are provided such that a portion of the elastic member 150 is hollowed in an axially central direction. Moreover, similar annular recesses 156 are provided between the intermediate plate 126 and the outer cylinder 124. The depths in the axial direction of these annular recesses 154 and 156 reach the locations of the expanding portion 130 and the curved surface portion 140, respectively. In addition, a length in an axial direction of the inner cylinder 122 is longer than the outer cylinder 124 and the intermediate plate 126.
With the structure as above, the spring in a direction A orthogonal to a central axis C of the inner cylinder 122 can be strengthened. Also, when the inner cylinder 122 and the outer cylinder 124 are inclined relative to each other through input of the twist, since the twist spring is decreased by the existence of the annular recesses 154,156, the inner cylinder 122 is easy to be inclined (hereinafter, the description shall be given such that the inner cylinder 122 is inclined relative to the outer cylinder 124).